1. Field of the Invention
The invention relates generally to a current-perpendicular-to-the-plane (CPP) magnetoresistive sensor that operates with the sense current directed perpendicularly to the planes of the layers making up the sensor stack, and more particularly to a CPP sensor with improved ferromagnetic layers.
2. Background of the Invention
One type of conventional magnetoresistive sensor used as the read head in magnetic recording disk drives is a “spin-valve” (SV) sensor. A SV magnetoresistive sensor has a stack of layers that includes two ferromagnetic layers separated by a nonmagnetic electrically conductive spacer layer, which is typically copper (Cu). One ferromagnetic layer has its magnetization direction fixed, such as by being pinned by exchange coupling with an adjacent antiferromagnetic layer, and the other ferromagnetic layer has its magnetization direction “free” to rotate in the presence of an external magnetic field. With a sense current applied to the sensor, the rotation of the free-layer magnetization relative to the fixed-layer magnetization is detectable as a change in electrical resistance.
In a magnetic recording disk drive SV read sensor or head, the magnetization of the fixed or pinned layer is generally perpendicular to the plane of the disk, and the magnetization of the free layer is generally parallel to the plane of the disk in the absence of an external magnetic field. When exposed to an external magnetic field from the recorded data on the disk, the free-layer magnetization will rotate, causing a change in electrical resistance. If the sense current flowing through the SV is directed parallel to the planes of the layers in the sensor stack, the sensor is referred to as a current-in-the-plane (CIP) sensor, while if the sense current is directed perpendicular to the planes of the layers in the sensor stack, it is referred to as current-perpendicular-to-the-plane (CPP) sensor. CPP-SV read heads are described by A. Tanaka et al., “Spin-valve heads in the current-perpendicular-to-plane mode for ultrahigh-density recording”, IEEE TRANSACTIONS ON MAGNETICS, 38 (1): 84-88 Part 1 JANUARY 2002.
The fixed or pinned ferromagnetic layer in a CPP-SV sensor used in read heads may be a single pinned layer or an antiparallel (AP) pinned structure. The AP-pinned structure has first (AP1) and second (AP2) ferromagnetic layers separated by a nonmagnetic antiparallel coupling (APC) layer with the magnetization directions of the two AP-pinned ferromagnetic layers oriented substantially antiparallel. The AP2 layer, which is in contact with the nonmagnetic APC layer on one side and the sensor's Cu spacer on the other side, is typically referred to as the reference layer. The AP1 layer, which is typically in contact with an antiferromagnetic or hard magnet pinning layer on one side and the nonmagnetic APC layer on the other side, is typically referred to as the pinned layer. If the AP-pinned structure is the “self-pinned” type, then no pinning layer is required. In a self-pinned structure where no antiferromagnet or hard magnet pinning layer is present, the AP1 layer is in contact with a seed layer on the sensor substrate. The AP-pinned structure minimizes magnetostatic coupling between the reference layer and the CPP-SV free ferromagnetic layer. The AP-pinned structure, also called a “laminated” pinned layer, and sometimes called a synthetic antiferromagnet (SAF), is described in U.S. Pat. No. 5,465,185.
In CPP-SV sensors, because the sense current flows perpendicular to all the layers in the sensor stack, the electrical resistance of the active region (the free layer, spacer layer and pinned layer) is a relatively small part of the total resistance of the sensor. For example, the antiferromagnetic layer, which is used for pinning the pinned layer and is not part of the active region, has a very high electrical resistivity and can account for more than 90% of the total stack resistance. It is thus desirable to increase the resistance of the active region without significantly increasing the total stack resistance. One approach to achieving this is sometimes called a confined-current-path (CCP) sensor, wherein the sense current is forced to pass though only a portion of the area of the sensor stack. One type of CCP CPP-SV sensor has a partially-oxidized nano-oxide layer (NOL) in the active region, typically in the conductive spacer layer. The sense current is confined to flow only though the conductive non-oxidized areas of the NOL. The NOL thus increases both the resistance and the AR of the active region and therefore increases the magnetoresistance (ΔR/R) of the sensor. CPP-SV sensors with NOLs are described by Oshima et al., “Current-perpendicular spin valves with partially oxidized magnetic layers for ultrahigh-density magnetic recording”, IEEE Transactions on Magnetics, Vol. 39, No. 5, September 2003, pp. 2377-2380; and by Fukuzawa, et al., “MR Enhancement by NOL Current-Confined-Path Structures in CPP Spin Valves”, IEEE Transactions on Magnetics, Vol. 40, No. 4, July 2004, pp. 2236-2238. However, because the formation of the conductive paths in the NOL is by oxidation and annealing of a very thin layer, the number and size of the conductive non-oxidized areas depends on the material properties, layer thickness, oxidation time, and anneal conditions. As a result it is difficult to reliably manufacture large quantities of CPP-SV sensors with NOLs with predictable values of R and ΔR/R. Moreover, the size and shape of the conductance paths has a statistical distribution that cannot be easily controlled, yielding unwanted large distributions for R and ΔR/R within a given group of devices.
The materials making up the free layer and the pinned layer (either the single pinned layer or the AP2 layer in an AP-pinned structure) are typically crystalline alloys of CoFe or NiFe. These materials have a relatively low electrical resistivity and thus do not contribute significantly to the resistance of the active region.
What is needed is a CPP-SV sensor with improved ferromagnetic materials for the free and pinned layers that increase the electrical resistance of the active region of the sensor without significantly degrading the sensor's magnetic properties.